1. Field of the Invention
The present invention is directed in general to field of user interfaces and user control of an electronic device. In one aspect, the present invention relates to a method, apparatus and system for determining the location of an object on a keypad or device having a touch sensitive surface.
2. Description of the Related Art
Touch panels are increasingly used in a variety of electronic devices, such as cellular telephones, appliances, industrial equipment, MP3 players, personal digital assistants (PDAs), tablet computers and other similar portable electronic devices. A touch panel typically includes an input touch screen, keypad or buttons for providing data entry, invoking applications, setting up system commands, and menu traversing. For example, cellular phones typically have a conventional pushbutton telephone keypad layout, with the addition of specialized keys for operating the cellular phone. Touch panels have advantages over conventional solutions since touch panels do not have complex mechanical parts that can be expensive to manufacture and can wear out or break. In addition, touch panels can be completely sealed to the external environmental conditions that can degrade the contacts or get inside the product provoking malfunctions.
Capacitive touch sensing techniques are often used with touch panels to detect a change in a signal due to capacitive coupling created by a touch on the touch panel. Such capacitive sensors include one or more electrodes in the touch panel below each keypad, where each electrode forms one capacitor plate and the object being detected (e.g., a human finger) forms the other capacitor plate. In operation, an electric field that is applied to electrodes below the touch panel is changed or altered in the vicinity of a detected touch because the touch capacitively couples with the electrode(s). The change in the field is detected and used to determine the touch location. With touch panel designs, there a number of design constraints that can limit the accuracy of touch detection and/or increase the processing or fabrication complexity. For example, larger electrodes are more sensitive to detecting electric field variations, but there are size constraints for keypads with certain types of electronic devices, such as portable media devices, mp3 players, cell phones, etc. In addition, when electrode sizes are maximized within a small keypad area (e.g., a 4 cm×4 cm area for a numeric key pad of 12 keys), a finger that presses an intended key will also press down on one or more surrounding keys, which complicates the detection process by requiring sensors with higher sensitivities, analog to digital interfaces with more resolution or accurate signal conditioning stages to be able to distinguish between a valid and an invalid touch. Other design constraints are posed by the type and thickness of dielectric material used to form a touch panel, since the variation of the capacitance c is proportional to the area A of two parallel plates times the dielectric constant k of the object between them and inverse to the distance d between the plates. These constraints can impose minimal and maximum electrode area requirements for providing a threshold level of detection accuracy with a typical 12 key numeric keypad configuration.
Accordingly, a need exists for an improved capacitive touch sensing keypad that addresses various problems in the art that have been discovered by the above-named inventors where various limitations and disadvantages of conventional solutions and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow, though it should be understood that this description of the related art section is not intended to serve as an admission that the described subject matter is prior art.